Spokes tension with hot rim

Upon sustained braking, with rim getting very hot, the spokes tension
cannot but increasing.
How much?

Sergio
Pisa

On Fri, 23 Jan 2009 22:52:53 -0800 (PST), sergio
wrote:

Upon sustained braking, with rim getting very hot, the spokes tension
cannot but increasing.
How much?

Sergio
Pisa

Dear Sergio,

Probably not enough to regain the spoke tension lost when the the tire
was mounted and inflated.

Most wheels are built and tensioned bare, so their riding tension ends
up noticeably lower because of the tire constricts when inflated.

(Cheap wide single-wall MTB rims actually work the other way, gaining
spoke tension with higher tire pressure, but I expect that we're
talking about typical narrow 700c rims here.)

Here's Dianne's page showing how increased tire pressure lowered spoke
tension:

http://www.geocities.com/dianne_1234...-inflation.htm

Dianne found that 120 psi lowered 95 kgf bare-tire spoke tension ~13%
to 81 kgf.

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.

Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

Cheers,

Carl Fogel

On 24 Gen, 09:01, wrote:

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.
Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

Dearest Carl,
far better than any thermo-mevchanical calculation
I would trust measurements (upon stopping the bike).

Sergio
Pisa

wrote:
On Fri, 23 Jan 2009 22:52:53 -0800 (PST), sergio
wrote:

Upon sustained braking, with rim getting very hot, the spokes tension
cannot but increasing.
How much?

Sergio
Pisa

Dear Sergio,

Probably not enough to regain the spoke tension lost when the the tire
was mounted and inflated.

Most wheels are built and tensioned bare, so their riding tension ends
up noticeably lower because of the tire constricts when inflated.

(Cheap wide single-wall MTB rims actually work the other way, gaining
spoke tension with higher tire pressure, but I expect that we're
talking about typical narrow 700c rims here.)

Here's Dianne's page showing how increased tire pressure lowered spoke
tension:

http://www.geocities.com/dianne_1234...-inflation.htm

Dianne found that 120 psi lowered 95 kgf bare-tire spoke tension ~13%
to 81 kgf.

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.

Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

So when the rim manufacturers quote spoke tension, are they quoting
tyre-on or tyre-off tensions? Do we care?

sergio pretended :
On 24 Gen, 09:01, wrote:

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.
Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

Dearest Carl,
far better than any thermo-mevchanical calculation
I would trust measurements (upon stopping the bike).

Sergio
Pisa

If you would trust measurement I suggest not to post your question in
this NG but get off your bike and measure!

;-)

Martin

On 24 Gen, 10:31, Martin Borsje wrote:
If you would trust measurement I suggest not to post your question in
this NG but get off your bike and measure!

Thanks.
I never stop until cooled off.

Sergio
Pisa

On 24 Gen, 16:15, Phil W Lee phil(at)lee-family(dot)me(dot)uk wrote:
It depends on how much heat is conducted into the spokes and expanding
them

Joke for joke.
.... and that depends on with what oil you lubed the nipples.
Plus, I guess you mean longitudinal expansion, right? The transversal
part should play a second order effect.

Sergio
Pisa

On Sat, 24 Jan 2009 00:10:06 -0800 (PST), sergio
wrote:

On 24 Gen, 09:01, wrote:

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.
Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

Dearest Carl,
far better than any thermo-mevchanical calculation
I would trust measurements (upon stopping the bike).

Sergio
Pisa

Dear Sergio,

I suspect that you'll have to find something with better resolution
than a hand-held bicycle-spoke tension-gauge to detect the change in
spoke tension between cold and hot rims.

Here are the results from a strain gauge on a wheel in action,
recording the rise and fall of spoke tension during a ride:
http://i39.tinypic.com/24b93xu.jpg

The spoke-tension loss from 95 kgf that Dianne measured when a tire
was inflated to 120 psi was ~130 on the scale in that graph.

It's from this paper:
http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf

Unfortunately, the photo of the actual test rig is blank.

Since no one else has taken a stab at calculating the result, here's
an ice-pick in the eye of science.

An inch of typical aluminum expands aroud 10 -6th x 12 per degree F,
according to this site:

http://metals.about.com/gi/dynamic/o...k/comparis.htm

A 700c rim is about 83 inches, so when heated from ~70F to ~250 F, it
should try to expand this much:

83 inch * 180 degrees F * 0.000012 inch/inch per degree F

0.17928 inches, or about 0.18 inches

So an 83.0 inch rim tries to expand to 83.18 inches, roughly 0.22%.

The diameter of the rim also tries to expand the same ~0.22%, and so
do the spokes (with some fudge factors thrown in).

If we try to lengthen a ~290 mm spoke 0.22%, it tries to become
~290.638 mm long.

Now we look at "The Bicycle Wheel" appendix, where "Spoke Elongation"
is worked out for us. We have to use the 3rd edition, which corrects a
typo (180 kg for 100 kg). For a 1.6 mm thick spoke, the elongation is
calculated to be 0.75 mm for 1000 newtons, or 0.75 mm for 102 kgf, or
~0.0075 mm of spoke stretch per kgf.

Unfortunately, 0.638 mm / 0.0075 mm/kgf = an implausible 85 kgf spoke
tension increase.

It's unlikely that spokes tensioned to ~85 kgf (with inflated tires)
double their tension due to rim heating on long downhills--the wheels
wouldn't be likely to reach the bottom.

Several explanations with a common theme suggest themselves.

A) The idiot doing the hasty calculations is off by a decimal place
somewhere, and the result should be a plausible ~8.5 kgf spoke tension
increase, not the implausbile ~85 kgf increase.

B) The idiot doing the hasty calculations is wasting his time with a
an over-simplified or completely mistaken model. The aluminum rim bed,
for example, will stretch inward, reducing how much the spokes stretch
outward. More likely, the idiot's assumptions about how things expand
against tension may have left engineers hospitalized with hernias from
uncontrollable laughter.

C) The idiot doing the hasty calculations is exquisitely correct and
should trust his figures instead of assuming that wheels would fail on
long downhills if rim heating doubled their spoke tension from ~85 kgf
to ~170 kgf. (But even an idiot finds this explanation hard to
swallow.)

Cheers,

Carl Fogel

wrote:
I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.
Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

sergio wrote:
far better than any thermo-mevchanical calculation
I would trust measurements (upon stopping the bike).

wrote:
I suspect that you'll have to find something with better resolution
than a hand-held bicycle-spoke tension-gauge to detect the change in
spoke tension between cold and hot rims.

Here are the results from a strain gauge on a wheel in action,
recording the rise and fall of spoke tension during a ride:
http://i39.tinypic.com/24b93xu.jpg

The spoke-tension loss from 95 kgf that Dianne measured when a tire
was inflated to 120 psi was ~130 on the scale in that graph.

It's from this paper:
http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf

Unfortunately, the photo of the actual test rig is blank.

Since no one else has taken a stab at calculating the result, here's
an ice-pick in the eye of science.

An inch of typical aluminum expands aroud 10 -6th x 12 per degree F,
according to this site:

http://metals.about.com/gi/dynamic/o...k/comparis.htm

A 700c rim is about 83 inches, so when heated from ~70F to ~250 F, it
should try to expand this much:

83 inch * 180 degrees F * 0.000012 inch/inch per degree F

0.17928 inches, or about 0.18 inches

So an 83.0 inch rim tries to expand to 83.18 inches, roughly 0.22%.

The diameter of the rim also tries to expand the same ~0.22%, and so
do the spokes (with some fudge factors thrown in).

If we try to lengthen a ~290 mm spoke 0.22%, it tries to become
~290.638 mm long.

Now we look at "The Bicycle Wheel" appendix, where "Spoke Elongation"
is worked out for us. We have to use the 3rd edition, which corrects a
typo (180 kg for 100 kg). For a 1.6 mm thick spoke, the elongation is
calculated to be 0.75 mm for 1000 newtons, or 0.75 mm for 102 kgf, or
~0.0075 mm of spoke stretch per kgf.

Unfortunately, 0.638 mm / 0.0075 mm/kgf = an implausible 85 kgf spoke
tension increase.

It's unlikely that spokes tensioned to ~85 kgf (with inflated tires)
double their tension due to rim heating on long downhills--the wheels
wouldn't be likely to reach the bottom.

Several explanations with a common theme suggest themselves.

A) The idiot doing the hasty calculations is off by a decimal place
somewhere, and the result should be a plausible ~8.5 kgf spoke tension
increase, not the implausbile ~85 kgf increase.

B) The idiot doing the hasty calculations is wasting his time with a
an over-simplified or completely mistaken model. The aluminum rim bed,
for example, will stretch inward, reducing how much the spokes stretch
outward. More likely, the idiot's assumptions about how things expand
against tension may have left engineers hospitalized with hernias from
uncontrollable laughter.

C) The idiot doing the hasty calculations is exquisitely correct and
should trust his figures instead of assuming that wheels would fail on
long downhills if rim heating doubled their spoke tension from ~85 kgf
to ~170 kgf. (But even an idiot finds this explanation hard to
swallow.)


How about (taking a theme from a concurrent thread) "within reasonable
and acceptable limits" since I can't cite a case of spoke pulling
through rim or taco rim as a result of brake heating.
--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

Tosspot wrote:
wrote:
On Fri, 23 Jan 2009 22:52:53 -0800 (PST), sergio
wrote:

Upon sustained braking, with rim getting very hot, the spokes tension
cannot but increasing.
How much?

Sergio
Pisa
Dear Sergio,

Probably not enough to regain the spoke tension lost when the the tire
was mounted and inflated.

Most wheels are built and tensioned bare, so their riding tension ends
up noticeably lower because of the tire constricts when inflated.

(Cheap wide single-wall MTB rims actually work the other way, gaining
spoke tension with higher tire pressure, but I expect that we're
talking about typical narrow 700c rims here.)

Here's Dianne's page showing how increased tire pressure lowered spoke
tension:

http://www.geocities.com/dianne_1234...-inflation.htm

Dianne found that 120 psi lowered 95 kgf bare-tire spoke tension ~13%
to 81 kgf.

I suspect that this is far more than any tension rise due to rim
heating, but someone else may come up with some interesting
calculations.

Such calculations would have to include the counter-effect of
increased tire constriction as the heated tire's pressure increases.

So when the rim manufacturers quote spoke tension, are they quoting
tyre-on or tyre-off tensions? Do we care?

I'm interested in the answer to this question too. What do the experts do?

Thanks

Martin
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